This invention relates to an ophthalmoscopic instrument equipped with means for detecting that the patient's eye is properly positioned with respect to the instrument.
In an ophthalmoscopic instrument, it is required that the instrument be positioned at a proper relationship with respect to the patient's eye. More specifically, it is necessary to make the optical axis of the instrument aligned with that of the patient's eye to establish an optical axis alignment, and to position the instrument at a predetermined distance from the patent's eye to establish a proper working distance adjustment. If the alignment and the working distance adjustment are not satisfactory in an eye fundus camera, for example, a part of the light projected to the patient's eye and reflected at the cornea is allowed to pass into the optical path of the instrument causing flaring and ghosting. Also, if the optical axes alignment and the working distance adjustment are imperfect in a refractometer, measurement errors will occur. A variety of devices have therefore been proposed for detecting the proper positional relationship between the patient's eye and the ophthalmoscopic instrument in the past.
As an example of the conventional devices for detecting the proper position for the eye being examined, an eye fundus camera is provided with an optical system by which a target mark is projected onto the cornea of the eye being examined, and the proper position of the eye is detected from the position and focusing of the image of the target mark reflected from the corneal surface of the eye. In the target mark projecting system of this type, however, the target mark projecting light beam is projected through a very small aperture so that the depth of focus is increased, and hence it is difficult to judge the focus of the projected target mark image accurately for the purpose of detecting the proper position of the eye being examined.